1. Field of the Invention
The present invention relates to an image sensor unit incorporated in an image scanner used for reading images printed on documents. The present invention also relates to an image scanner incorporating such an image sensor unit.
2. Description of the Related Art
An example of conventional image scanner is shown in FIG. 10 of the accompanying drawings. The illustrated image scanner is a flat-bed type scanner including an image sensor unit 9 arranged under a transparent document supporting panel 90. The image sensor unit 9 is movable in the secondary scanning direction indicated by a two-headed arrow Na. The image sensor unit 9 is provided with a light source 91, a plurality of reflectors or mirrors 92a-92c, a lens unit 93 for focusing images, and a line sensor 94 using CCDs (charge-coupled devices). These components are accommodated in a casing 95. The first and the second mirrors 92a, 92b are vertically spaced from each other in facing relation. As illustrated, light will be reflected repeatedly between these two mirrors.
In use, a document D is placed on the document supporting panel 90 to be illuminated by light from the light source 91. Then, the light reflected on the document D is guided by the mirrors 92a-92c to be brought to the lens unit 93. Finally, the lens unit 93 focuses the light onto the line sensor 94. During the image reading operation, the image sensor unit 9 is moved in the secondary scanning direction for reading images on the document line by line.
As described above, the first and the second mirrors 92a, 92b are used for causing the light reflected on the document D to travel along a zigzag path. Such an arrangement is advantageous in providing a long optical path extending from the document to the line sensor 94. As the optical path increases in length, the depth of field of the optical system widens. An increased depth of field makes it possible for the scanner to perform a proper image reading operation (which results in reproduction of distinct images) even when the document fails to be held in close contact with the document supporting panel 90.
Though the conventional image scanner has the above merits, it also has the following disadvantages.
Specifically, in the conventional image scanner, the first and the second mirrors 92a, 92b are vertically spaced by a distance H within the casing 95. In such an arrangement, when a much longer optical path is desired, the distance H may need to be increased. However, when the distance H is increased, the vertical dimension or thickness T of the casing 95 is also increased. In this connection, it should be noted that each of the first and the second mirrors 92a, 92b needs to be substantially equal in length to the maximum read span for the document D. Thus, when these two mirrors 92a, 92b are spaced further away from each other, not just part of the casing 95 but the entirety of the casing 95 is increased in thickness, which is unfavorable for producing a compact image scanner.
The present invention has been proposed under the above circumstances and its object is to reduce or even eliminate the problems described above. For attaining this object, the present invention adopts the following technical measures.
According to a first aspect of the present invention, there is provided an image sensor unit for reading a document comprising:
a casing elongated in a primary scanning direction;
a light source for illuminating the document;
at least two mirrors held in facing relation to each other for alternately reflecting light therebetween;
a lens unit for focusing light reflected on the document; and
a sensor for receiving the light focused by the lens unit;
wherein said two mirrors are spaced from each other in a secondary scanning direction perpendicular to the primary scanning direction.
In a preferred embodiment, the lens unit and the sensor may be arranged below said two mirrors.
The image sensor unit may further comprise a first additional mirror for leading the alternately reflected light to the lens unit, the first additional mirror being smaller in length than said two mirrors.
The image sensor unit may further comprise a second additional mirror associating with the first additional mirror for leading the alternately reflected light to the lens unit.
Preferably, the second additional mirror may be similar in length to the first additional mirror.
According to another preferred embodiment, the lens unit and the sensor may be spaced from each other in the primary scanning direction.
According to still another embodiment, said two mirrors may be held in nonparallel relation to each other.
In this embodiment, an angle between a vertical line and one of said two mirrors may be different from another angle between another vertical line and the other of said two mirrors.
Preferably, the light source may comprise a cold-cathode tube. Alternatively, it may be a xenon lamp.
The image sensor unit may further comprise a power supplying device for the light source. The power supplying device may be arranged under said two mirrors and attached to the casing.
According to a second aspect of the present invention, there is provided an image scanner for reading a document comprising:
an image sensor unit which includes: a casing elongated in a primary scanning direction; a light source for illuminating the document; at least two mirrors held in facing relation to each other for alternately reflecting light therebetween, said two mirrors being spaced from each other in a secondary scanning direction perpendicular to the primary scanning direction; a lens unit for focusing light reflected on the document; and a sensor for receiving the light focused by the lens unit;
a document supporting panel arranged above the image sensor unit; and
a driving mechanism for moving the image sensor unit in the secondary scanning direction.
In a preferred embodiment, the casing of the image sensor unit may be provided with a downwardly protruding portion and end portions adjacent to the protruding portion.
Preferably, the driving mechanism may include a drive motor, a first pulley connected to the drive motor, a second pulley spaced from the first pulley in the secondary scanning direction, and an endless belt engaging with the first and the second pulleys, wherein the endless belt may be arranged under one of the end portions of the casing.
The image scanner may further comprise first and second guide means for guiding the image sensor unit moving in the secondary scanning direction, the first and the second guide means being arranged under the end portions of the casing.
Preferably, the first guide means may include a guide rod extending in the secondary scanning direction.
On the other hand, the second guide means may include a hollow box-shaped member elongated in the secondary scanning direction.
Preferably, the image scanner may further comprise a printed circuit board for controlling the image sensor unit, the printed circuit board being enclosed by the box-shaped member.
Other objects, features and advantages of the present invention will become clearer from the following detailed description given with reference to the accompanying drawings.